Large amounts of dyes are used in various industrial fields, such as food, drug, cosmetic, textile and tanning fields (McMullan et al., 2001). It is estimated that the annual world production of dyes is above 700,000 tons, more than a half of which include dyes for textile fibers, 15% are dyes for other substrates such as leather and paper, 25% are organic pigments and the remaining portion is made up of dyes for particular uses (McMullan et al., 2001, Pearce et al., 2003).
Depending on molecule charge, dyes can be classed into anionic (acid), cationic (basic) and non-ionic dyes. As an alternative, depending on the chromophore group they can be classed into azo, anthraquinone, indigo, stilbene dyes etc., or depending on their applications. Azo and anthraquinone dyes represent the most widespread classes of dyes for industrial applications (Soares et al., 2001). Azo dyes are characterized by the presence of a double bond N═N and by other groups that are hard to degrade (Martins et al., 2001) and represent more than 50% of total production. Their fixing capacity is generally low and so more than 40% of the amount used gets into industrial waste, which has a clear color resulting therefrom, even after accurate purification treatments (O'Neill et al., 1999). Anthraquinone dyes represent the second class for industrial relevance and can be divided into dyes derived from indigo and from anthraquinone. They are prepared by successive introduction of the substituents on the pre-formed skeleton of anthraquinone.
Every year 5% to 10% of the world production of textile dyes is discharged into industrial wastewaters, which get in their turn into natural waterways where they can cause great problems for the environment and for living organisms (Yesilada et al., 2003). As a matter of fact, conventional methods for treating wastewaters are not sufficient to completely remove most of the dyes, which therefore tend to accumulate in the environment due to their complex molecular structure, designed on purpose for giving high stability to light, water and oxidizing agents (Fu and Viraraghavan, 2002a).
Dyes are toxic substances as shown by ETAD (1989) in a test on animals for 4,000 dyes. They can also have a carcinogenic and mutagenic action, due to the formation of aromatic amines when they are degraded under anaerobiosis from bacteria, as was shown in several researches on fishes, mice and other animals (Weisburger et al., 2002). Genotoxic and carcinogenic effects are also possible on men, on whom dyes cause at least short-term phenomena of contact and inhaling irritation (Yesilada et al., 2003).
When dyes get into surface water, indirect damages to ecosystems are likewise serious. As a matter of fact, gas solubility is compromised and above all water transparency properties are altered, which results in serious consequences for flora and fauna (Fu and Viraraghavan, 2002a). Lower penetration of sun rays causes indeed a reduction of oxygen concentration, which can be in its turn fatal for most water organisms (Yesilada et al., 2003).
Toxic substances contained in waste of industries using dyes should therefore be completely removed before being released into the environment (Knapp et al., 2001). Physical and chemical purification methods are not always applicable and/or effective and always involve high costs for firms (Fu and Viraraghavan, 2001, Robinson et al., 2001).
Chemical treatments exploiting oxidizing processes are among the most used methods, above all thanks to their easy application. Some of them, however, involve the use of chemical compounds that are noxious for men's health and/or for the environment such as the use of bleaching agents (Knapp et al., 2001). Among the most widespread treatments the following should be mentioned: treatment with H2O2 together with iron salts, with sodium hypochlorite, with ozone, photochemical and photocatalytic methods, electrochemical destruction (Robinson et al., 2001).
Physical methods based on the absorption of dyes into various abiotic matrices have proved to be effective in many cases. Decolourization by absorption is mainly based on ion exchange, which is affected by several factors such as the interaction between the dye and the type of substances used for absorption, temperature, pH, contact time, etc. Active carbons, peat, wood chips, filtration membranes are the most used absorbing agents. Absorption is often favored by the use of ultrasounds (Robinson et al., 2001, Crini, 2006).
A valid alternative to most traditional treatments of dyed wastewaters, characterized by low cost and low environmental impact, is the use of biologic systems, i.e. biomasses that are able to degrade toxic substances up to the mineralization thereof (biodegradation), or absorb them more or less passively on their cell structures (biosorption) (Banat et al., 1996).
Recently, several researches have shown that biosorption can be regarded as a valid alternative to chemical-physical methods and to microbial and/or enzymatic biodegradation. Such researches have pointed out the capacity of various microbial biomasses (bacteria, yeasts, fungi and algae) to absorb or accumulate dyes (Polman et al., 1996, Crini, 2006), and among the various types of biomass the fungal biomass has proved to be particularly suitable, even if the mechanisms regulating absorption have not yet been fully explained (Knapp et al., 2001, Crini, 2006).
In studies on biosorption with fungal biomasses, Mitosporic fungi and Zygomycetes, belonging to the genus Aspergillus, Penicillium, Myrothecium and Rhizopus, are mainly used. Only in some cases Basidiomycetes are used, since for these fungi the main decolourization mechanism is degradation and, according to Knapp et al. (2001), absorption occurs only in the initial stage of fungus-dyes interaction, which allows to create a strong contact between chromophores and degrading enzymes associated to the surface of hyphae.
Mechanisms regulating dye biosorption by the biomass seem to vary both as a function of the chemical structure of the dye and as a function of the specific chemical and structural composition of the biomass used. As a matter of fact, it was shown that some dyes have a particular affinity for particular species of organisms (Robinson et al., 2001).
Fu and Viraraghavan (2002b), working with biomasses of Aspergillus niger that had been deactivated, dried, pulverized and subjected to various chemical treatments, so as to selectively deactivate different chemical groups, have shown that dye biosorption preferably occurs on cell wall, where the main binding sites would be made up of amine and carboxyl groups. It should still be explained whether during biosorption processes the dye is bound only to the outer surface or whether it can also be carried, at least partially, into the hyphae (Polman and Breckenbridge, 1996; Brahimihorn et al., 1992).
With respect to traditional chemical-physical methods, biosorption has indubitable advantages such as a highly rapid treatment and the possibility of recovering absorbed dye for future use. Moreover, it can be carried out also with deactivated biomasses; this has huge advantages both thanks to the lower environmental impact and because it is not necessary to monitor the various factors affecting the growth of a living organism.
However, there are several factors that might affect biosorption yields, in particular growth substrate, pH, incubation temperature and initial dye concentration (Aksu and Tezer, 2000; Abd El Rahim et al., 2003, Aksu Z., 2005).